Recently, governments, law enforcement agencies, and investigators have warned of possible terrorist attacks involving radioactive materials. A so-called “dirty bomb” threat exists that could terrorize a population center by scattering Cesium 137 powder with conventional explosives. Talcum powder light, radioactive Cesium would disperse into the air to contaminate a wide area creating a hot-zone nightmare.
Radioactive materials are also used in a wide range of legitimate activities, from medical research and treatments to industrial processing and testing. Even using radioactive material in a legitimate fashion, people have been injured by accidental exposures to industrial radioisotopes. Loss of medical radiation sources is a constant concern.
Detecting the presence of radioactive materials is thus an important component of public safety. For example, it would be useful to know if terrorists were using a facility to stage an attack or to hide a dirty bomb. It would also be important to detect whether an industrial radioactive source was inadvertently exposing personnel to gamma radiation. A device to detect whether criminals are using ports or other transportation facilities to move radioactive or fissile materials, whether first-responders to an incident have entered a hot zone at the site of a bomb blast, or whether police officers have unknowingly stopped a vehicle carrying radioactive contraband would also be an important contributor to public safety.
The current radiation detection art comprises two methods. In the first, personnel who otherwise suspect the presence of radiation, perhaps by circumstance, use portable hand-held radiation survey meters to manually scan a suspected radioactive area. Typically these devices are battery operated, or are rechargeable, and are deployed only when a particular suspicion is raised. Because the equipment is rarely used it tends to be neglected. Thus, when needed most, the monitoring equipment is usually unavailable to respond to an emerging situation or threat.
If radiation equipment is permanently installed to monitor a defined area, the equipment must deal with background radiation levels, which may change slowly over time. Background radiation levels are typically low and erratic, such that it is difficult to discern when small changes in the rate occur. Conventional radiation monitors use long-time-constant low-pass filters to track the average rate. To achieve finer resolution longer time constants are employed. The simplest method of filtering is to count the number of events in a period of many seconds or minutes to compute the average rate. Such use of low-pass filtering or counting techniques is not well suited to homeland security applications where the offending radioactive source may be present for only a brief period of time.
In the second method, individuals who work in areas that have a perceived higher risk for radiation exposure wear a portable radiation dosimeter which sums the radiation exposure to the device. A disadvantage to wearable dosimetry is that specialized training and equipment is generally required to read the radiation dose gathered by the dosimeter, usually hours or days after the exposure. An individual may be able to assess the past radiation exposure with a dosimeter, but the knowledge would come too late to avert or respond to a transient incident. In addition, specially trained operators are required to judge whether the levels of radiation are indeed unusual or are the result of some explainable, legitimate, activity.
Thus, what is needed is a method and apparatus which can provide long-term monitoring for radiation, with high accuracy and specificity, and which can detect and respond as quickly as possible to a transient radiation incident. The effectiveness of such an apparatus is significantly enhanced if it could be implemented in an inexpensive and compact package that can be widely distributed.